In continuous ink jet printing, streams are discharged from an orifice or array of orifices to form droplet streams. To regulate the streams' breakup into uniformly sized and spaced drops, series of energy pulses of predetermined frequency are applied to the ink stream. One preferred mode for applying the pulse series that stimulate uniform droplet streams is by vibration, e.g., for the orifice plate, a resonator housing or the ink volume behind the orifices. When the issuing ink streams (called filaments) break up properly into droplet streams, the filament tip separates into a droplet within a predetermined drop charge region that is opposite a charge electrode. The charge electrode is energized with a charge voltage, or is not so energized, in accord with an information signal; and because the ink is conductive and grounded, a charge is correspondingly induced, or not induced, on the drop then formed at the drop charge region. Ink droplets thereafter pass to the print zone, or are caught, in accord with their charged or non-charged conditions.
It will be appreciated that one important factor for good printing operations is that the ink filaments break up into drops within a range of locations along the drop path that is acceptably close to the charge electrode. The nominal charging region is defined mainly by the length of the drop charge electrode in the direction of the ink jet stream. Drop breakup before or beyond the nominal charging region can result in improper drop charging.
To obtain reliable drop charging, it is also important that the stimulating energy applied to the ink filament have an amplitude that avoids formation of small satellite drops during drop break off. It is difficult to control drop charging and deflection in the presence of such satellite drops. It is well known, e.g., see U.S. Pat. No. 4,631,549, that as the amplitude of drop-stimulating energy increases, the drop break off conditions change from: (i) "underdrive" conditions where satellites are formed to (ii) a satellite-free condition where no satellites are formed to (iii) an "overdrive" condition where satellites are again formed. In addition to dependence on stimulation energy amplitudes, the domains where satellite and non-satellite drop formation occur depend on other systems parameters (e.g., ink temperature, ink viscosity and ink pressure). These parameters will vary gradually over periods of time and prior art techniques have been developed to periodically check and adjust the stimulation amplitude to assure optimum drop charging.
In one prior art procedure, the underdrive and overdrive drop formation conditions are visually identified, and an operating stimulation amplitude between the amplitudes corresponding to those conditions is selected. The visual adjustment technique requires high skill levels in detecting and measuring the critical conditions and it is very time consuming.
In a procedure taught by U.S. Pat. No. 4,631,549, an operating stimulation amplitude is adjusted by detecting the stimulation amplitude at the infinite satellite condition (with an electrometer) and selecting the operation amplitude to be a value that is a predetermined multiple of the detected infinite satellite stimulation amplitude. The infinite satellite detection and adjustment approach is sometimes hard to effect, e.g., when satellites are smaller than normal due to lower ink pressures. Also, the optimum operating point amplitude sometimes varies relative to the infinite satellite amplitude from a fixed predetermined multiple value, depending, e.g., on temperature, pressure, orifice size and ink properties.
Another approach for adjusting the stimulation amplitude is described in a publication "Servo Control of Multinozzle Ink-Jet Operating Point"by G. L. Ream; IEEE Transactions on Industry Applications, Vol. IA-20, No. 2, March/April 1984. In this approach, the stimulation amplitude is set to operate at the minimum filament length condition by detection of changes in relative phase between drop break off and a drop charge signal. For each of a series of stimulation amplitude increases a 20% duty cycle pulse train is incremented through 16 phase settings, and drop charge is measured to determine a mean break off phase. When the mean break off phase reverses sign, the minimum filament length has been reached. This procedure is time consuming and electronically complicated.